1. Field
The present invention relates to a tank arrangement for a marine structure. The tank arrangement is adapted for a submersible pump, the tank arrangement comprises a circulation conduit for circulating liquid to enable the submersible pump to operate in an advantageous condition substantially at all time. The present invention also relates to an assembly of a submersible pump and a tank arrangement.
2. Background of the Invention
A marine structure, such as a ship or a semi-submersible unit, is often provided with one or more ballast systems in order to control the draught and/or the inclination of the marine structure. Generally, a ballast system comprises a ballast tank, and in fact often a plurality of tanks, which is adapted to be filled with sea water—i.e. water ambient of the marine structure—through a water filling assembly.
In order to be able to empty or to transfer water between tanks, the ballast system further generally comprises a pump assembly which in turn comprises a pump and means for fluidly connecting the tank and the pump as well as means for connecting the pump and the environment ambient of the marine structure such that water may be pumped from the tank to the ambient environment. Generally, at least a portion of the pump assembly is in fluid communication with the aforesaid water filling assembly.
However, in some operations, intrusion of air cannot be avoided. For example, when a ballast tank is drained of water, there is a risk that air, at low tank level in the ballast tank, will be mixed with ballast water and guided towards to, and later introduced into the pumps such that air will be entrained in the water filling assembly and—at a later stage—in at least a portion of the pump assembly. As such, when a ballast tank is to be emptied of water, there is a risk that the air in the pump assembly will be guided towards the pump and hence introduced in the pump. Since air generally adversely affects a pump, the presence of air is undesired. Moreover, at the completion of a ballast tank emptying operation, i.e. when a ballast tank is almost completely emptied of water, the water flow from the ballast tank to the pump is generally lower than in the beginning of the ballast tank emptying operation. Since a pump generally has an optimum operating condition at a specific combination of the flow rate and pressure, the aforesaid change in the water flow is generally undesired.
Additionally, during load altering operations of the marine structure, such as multiple ballast operations and/or oil refueling, which occur simultaneously as a ballast tank emptying operation, there may be a need for controlling the rate at which the ballast tank is emptied in order to maintain a balance in the marine structure. Moreover, when a ballast tank is almost emptied of water, it may be desirable to have a low flow rate of the water leaving the ballast tank in order to at least limit the amount of air in the water entering the pump.
Pumping ballast water usually requires relatively high capacity pumps while when pumping bilge water, generally low capacity pumps are required. The difference in requirements of the pumps tends to be burdensome for the manufacturer of the marine structure as it demands two different pumps, each specifically adapted for the operating condition required for the specific purpose of the pump. Further, if the marine structure is damaged, it remains vital that the ballast pumps and bilge water pumps are operational, even if the hull is breached or the space at which the pumps are positioned is flooded. Today, this is often solved by having a plurality of pump rooms at different locations; this ensures that always one ballast pump or bilge water pump remains operational.
In view, of the above, it may be realized that there is a need for improvements in the field of moving fluids.